a/c acting very strange

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Ok this is the first of two problems and many, many questions that I need help with...
I don't think that this problem is vehicle specific, but for information sake, I will give as much background/basic info as I can.

1996 GMC Jimmy 4WD 4.3 L Vortec V6
most likely between 136,000 and 200,000 miles (odometer discrepancy)
only had the SUV for 2 weeks now, problem arose last weekend

Ok, now the problem...One afternoon last weekend, I got in the truck, rolled down all 4 windows (like I always do) to cool off the inside, turned on the a/c and proceeded to leave the parking lot. When I switched the knob from OFF to Max A/C, I heard a hissing noise (sounds like an air hose with a small hole in it) from under the bottom of the dash somewhere right of center or maybe right in front of the passenger seat, not sure. I began to cycle the knob through all of the settings. I noticed that I only hear that noise when it's set on Max A/C (with the blower on any speed).
At that point, it appeared that everything was functioning normally, so I ignored it for the moment. Shortly after, I hit the on ramp to the interstate and began to accelerate fairly quickly. As I did this I quickly noticed another problem. The air coming from the front dash vents slowed dramatically. The air coming from those vents (what little was coming out at that point) suddenly turned hot for a couple of seconds. And then air ceased coming from those vents all together and I noticed that the air was coming from the top (window/defrost) vents. Well just after I got onto the Hwy, I let off the accelerator to level off my speed. When I did this the air flow moved back to the front vents and resumed normal operation.
Since then I still hear the noise under the dash and the air flow cuts out and moves to the top vents whenever the motor experiences a heavier-than-normal payload (going up hills and all). Not sure if this will still happen while the vehicle is stationary and just revving the engine, will check that tomorrow.

Now the next section is a description of a problem (the second one, that I mentioned above) that I am not sure whether it is related or not. So, if it is determined that it's not, please ignore and I will take this up in a later thread.
Well the morning after this a/c problem arose, the "Service Engine Soon" light came on when I started the vehicle for the first time that day. That afternoon, after work, I went to my local auto parts store and asked them to retrieve the code related to this light. P0446 EVAP Emission Vent Control Malfunction is what they gave me. The Haynes Repair Manual for this vehicle defines this as "EVAP Canister Vent Blocked". From this, I assume that the two events are not related, but I don't really know enough to say for sure, so I don't discount it.

Now, I won't attempt anyone as to my knowledge/experience with auto-mechanics (as if it isn't obvious at this point). I have done all of the normal maint. on most of my vehicles in the past, and I have done things like change breaks, alternators, water pumps, fuel pumps, and such before. But, I have never really tackled anything like this. I do believe that I can learn to do anything that I want to do, but I also want to consider whether this will be more costly than taking it to a shop and have them do it.

Any help/advise/guidance will be deeply appreciated.

posted by  JJ Westbrook

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